Measurement of high speed signal strength often is required in wireless communication systems. A circuit commonly used for this purpose is an RMS-to-DC converter. Signal strength is measured by a detector using a square cell. An application of such a detector is described in my copending application Ser. No. 11/493,528, filed on Jul. 26, 2006, and titled “Low Power Wide Dynamic Range RMS-to-DC Converter, assigned to the common assignee and incorporated herein by reference. FIG. 1 shows an open-loop power detector of a general type implemented in the co-pending application. The power measurement of an input signal can be best described in the following equation (1), where a square cell is needed before average power determination is performed.
                                                                        V                out                            =                                                1                  T                                ⁢                                                      ∫                    0                    T                                    ⁢                                                            I                      out                                        ⁢                                                                                  ⁢                                          ⅆ                      t                                        *                    R                                                                                                                          =                                                K                  1                                *                                  1                  T                                *                                                      ∫                    0                    T                                    ⁢                                                            V                                              i                        ⁢                                                                                                  ⁢                        n                                            2                                        ⁢                                                                                  ⁢                                          ⅆ                      t                                        *                    R                                                                                                                          =                                                K                  2                                *                                  P                                      i                    ⁢                                                                                  ⁢                    n                                                                                                          (        1        )            To provide a signal strength measurement that is independent of any particular input signal being processed, the detector should be insensitive to input waveforms, even those with high crest factor, or peak amplitude divided by RMS value.
Optimal operation is dependent upon several factors. Precision measurement of input signal regardless of its waveform over a wide dynamic range of the input signal power is an important consideration. Fast envelope detection of a modulated input signal is advantageous in obtaining an output that is independent of input signal waveform. Linear input impedance as a function of input power is needed to avoid distortion back to the signal source. As systems often may be subject to considerable temperature variations, stable output should be maintained across the operating temperature range of the circuit. Low DC power consumption, of course, also is an important objective. Accordingly, a square cell having these desirable characteristics is needed for the application described as well as for other applications.